Opposite of fast fashion (rapidly created clothing based off catwalk trend mass produced in vast amounts, normally in very poor working conditions, see Primark, H&M, Zara, GAP, Uniqlo, TopShop etc). Cheap, generally fairly low quality, aim is to encourage people to buy more and discard their "old" clothes.

So slow fashion aims to be the opposite, to slow consumerism (sustainable working conditions, materials, emphasis on locality where possible, etc). Obviously more expensive as new clothes removes the mass production [sweatshop] element and uses higher quality materials that won't fail as quickly. As sibling comment says second hand clothes are major part of this

I imagine it has to do with second hand clothing stores/goodwill, but I could be wrong.